If the grounds are not connected then enabling the internal pull ups is not going to help.You only need to enable the internal pull up if you have an open collector output on your other device.This is written for power supplies but it applies equally to signals:-http://www.thebox.myzen.co.uk/Tutorial/Power_Supplies.html

with regards to connecting grounds, we connected the ground of the supply of our input device to the ground of our arduino... but yet there's also a ground near the outputs of our input device...and i assume the 2 grounds are connected, so we did not connect it

Is there a device connected to pin 12? Is it turned on? Is it wired correctly? Perhaps a photo or schematic?

i don't have a photo or a schematic sir, but it is an infrared device which is controlled by a remote in which when a corresponding button in the remote is pressed, it has digital output, which is connected to pin 12. my problem is, even though i'm not pressing a button, the serial monitor outputs infinite loop of high and low, which really confuses me if this is a pull down resistor issue.